Early Retransmit for TCP

and Stream Control Transmission Protocol (SCTP)

Abstract

This document proposes a new mechanism for TCP and Stream Control
Transmission Protocol (SCTP) that can be used to recover lost
segments when a connection's congestion window is small. The "Early
Retransmit" mechanism allows the transport to reduce, in certain
special circumstances, the number of duplicate acknowledgments
required to trigger a fast retransmission. This allows the transport
to use fast retransmit to recover segment losses that would otherwise
require a lengthy retransmission timeout.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5827.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

1. Introduction

Many researchers have studied the problems with TCP's loss recovery
[RFC793, RFC5681] when the congestion window is small, and they have
outlined possible mechanisms to mitigate these problems
[Mor97, BPS+98, Bal98, LK98, RFC3150, AA02]. SCTP's [RFC4960] loss
recovery and congestion control mechanisms are based on TCP, and
therefore the same problems impact the performance of SCTP
connections. When the transport detects a missing segment, the
connection enters a loss recovery phase. There are several variants
of the loss recovery phase depending on the TCP implementation. TCP
can use slow-start-based recovery or fast recovery [RFC5681], NewReno
[RFC3782], and loss recovery, based on selective acknowledgments
(SACKs) [RFC2018, FF96, RFC3517]. SCTP's loss recovery is not as
varied due to the built-in selective acknowledgments.

All of the above variants have two methods for invoking loss
recovery. First, if an acknowledgment (ACK) for a given segment is
not received in a certain amount of time, a retransmission timer
fires, and the segment is resent [RFC2988, RFC4960]. Second, the
"fast retransmit" algorithm resends a segment when three duplicate

ACKs arrive at the sender [Jac88, RFC5681]. Duplicate ACKs are
triggered by out-of-order arrivals at the receiver. However, because
duplicate ACKs from the receiver are triggered by both segment loss
and segment reordering in the network path, the sender waits for
three duplicate ACKs in an attempt to disambiguate segment loss from
segment reordering. When the congestion window is small, it may not
be possible to generate the required number of duplicate ACKs to
trigger fast retransmit when a loss does happen.

Small congestion windows can occur in a number of situations, such
as:

(1) The connection is constrained by end-to-end congestion control

when the connection's share of the path is small, the path has a
small bandwidth-delay product, or the transport is ascertaining
the available bandwidth in the first few round-trip times of slow
start.

(2) The connection is "application limited" and has only a limited

amount of data to send. This can happen any time the application
does not produce enough data to fill the congestion window. A
particular case when all connections become application limited
is as the connection ends.

(3) The connection is limited by the receiver's advertised window.

The transport's retransmission timeout (RTO) is based on measured
round-trip times (RTT) between the sender and receiver, as specified
in [RFC2988] (for TCP) and [RFC4960] (for SCTP). To prevent spurious
retransmissions of segments that are only delayed and not lost, the
minimum RTO is conservatively chosen to be 1 second. Therefore, it
behooves TCP senders to detect and recover from as many losses as
possible without incurring a lengthy timeout during which the
connection remains idle. However, if not enough duplicate ACKs
arrive from the receiver, the fast retransmit algorithm is never
triggered -- this situation occurs when the congestion window is
small, if a large number of segments in a window are lost, or at the
end of a transfer as data drains from the network. For instance,
consider a congestion window of three segments' worth of data. If
one segment is dropped by the network, then at most two duplicate
ACKs will arrive at the sender. Since three duplicate ACKs are
required to trigger fast retransmit, a timeout will be required to
resend the dropped segment. Note that delayed ACKs [RFC5681] may
further reduce the number of duplicate ACKs a receiver sends.
However, we assume that receivers send immediate ACKs when there is a
gap in the received sequence space per [RFC5681].

[BPS+98] shows that roughly 56% of retransmissions sent by a busy Web
server are sent after the RTO timer expires, while only 44% are
handled by fast retransmit. In addition, only 4% of the RTO timer-
based retransmissions could have been avoided with SACK, which has to
continue to disambiguate reordering from genuine loss. Furthermore,
[All00] shows that for one particular Web server, the median number
of bytes carried by a connection is less than four segments,
indicating that more than half of the connections will be forced to
rely on the RTO timer to recover from any losses that occur. Thus,
loss recovery that does not rely on the conservative RTO is likely to
be beneficial for short TCP transfers.

The limited transmit mechanism introduced in [RFC3042] and currently
codified in [RFC5681] allows a TCP sender to transmit previously
unsent data upon receipt of each of the two duplicate ACKs that
precede a fast retransmit. SCTP [RFC4960] uses SACK information to
calculate the number of outstanding segments in the network. Hence,
when the first two duplicate ACKs arrive at the sender, they will
indicate that data has left the network, and they will allow the
sender to transmit new data (if available), similar to TCP's limited
transmit algorithm. In the remainder of this document, we use
"limited transmit" to include both TCP and SCTP mechanisms for
sending in response to the first two duplicate ACKs. By sending
these two new segments, the sender is attempting to induce additional
duplicate ACKs (if appropriate), so that fast retransmit will be
triggered before the retransmission timeout expires. The sender-side
"Early Retransmit" mechanism outlined in this document covers the
case when previously unsent data is not available for transmission
(case (2) above) or cannot be transmitted due to an advertised window
limitation (case (3) above).

Note: This document is being published as an experimental RFC, as
part of the process for the TCPM working group and the IETF to assess
whether the proposed change is useful and safe in the heterogeneous
environments, including which variants of the mechanism are the most
effective. In the future, this specification may be updated and put
on the standards track if its safeness and efficacy can be
demonstrated.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

The reader is expected to be familiar with the definitions given in
[RFC5681].

3. Early Retransmit Algorithm

The Early Retransmit algorithm calls for lowering the threshold for
triggering fast retransmit when the amount of outstanding data is
small and when no previously unsent data can be transmitted (such
that limited transmit could be used). Duplicate ACKs are triggered
by each arriving out-of-order segment. Therefore, fast retransmit
will not be invoked when there are less than four outstanding
segments (assuming only one segment loss in the window). However,
TCP and SCTP are not required to track the number of outstanding
segments, but rather the number of outstanding bytes or messages.
(Note that SCTP's message boundaries do not necessarily correspond to
segment boundaries.) Therefore, applying the intuitive notion of a
transport with less than four segments outstanding is more
complicated than it first appears. In Section 3.1, we describe a
"byte-based" variant of Early Retransmit that attempts to roughly map
the number of outstanding bytes to a number of outstanding segments
that is then used when deciding whether to trigger Early Retransmit.
In Section 3.2, we describe a "segment-based" variant that represents
a more precise algorithm for triggering Early Retransmit. This
precision comes at the cost of requiring additional state to be kept
by the TCP sender. In both cases, we describe SACK-based and non-
SACK-based versions of the scheme (of course, the non-SACK version
will not apply to SCTP). This document explicitly does not prefer
one variant over the other, but leaves the choice to the implementer.

3.1. Byte-Based Early Retransmit

A TCP or SCTP sender MAY use byte-based Early Retransmit.

Upon the arrival of an ACK, a sender employing byte-based Early
Retransmit MUST use the following two conditions to determine when an
Early Retransmit is sent:

Note that in the byte-based variant of Early Retransmit, "ownd"
is equivalent to "FlightSize" (defined in [RFC5681]). We use
different notation, because "ownd" is not consistent with
FlightSize throughout this document.

Also note that in SCTP, messages will have to be converted to
bytes to make this variant of Early Retransmit work.

(2.b) There is either no unsent data ready for transmission at the

sender, or the advertised receive window does not permit new
segments to be transmitted.

When the above two conditions hold and a TCP connection does not
support SACK, the duplicate ACK threshold used to trigger a
retransmission MUST be reduced to:

ER_thresh = ceiling (ownd/SMSS) - 1 (1)

duplicate ACKs, where ownd is expressed in terms of bytes. We call
this reduced ACK threshold enabling "Early Retransmission".

When conditions (2.a) and (2.b) hold and a TCP connection does
support SACK or SCTP is in use, Early Retransmit MUST be used only
when "ownd - SMSS" bytes have been SACKed.

If either (or both) condition (2.a) and/or (2.b) does not hold, the
transport MUST NOT use Early Retransmit, but rather prefer the
standard mechanisms, including fast retransmit and limited transmit.

As noted above, the drawback of this byte-based variant is precision
[HB08]. We illustrate this with two examples:

Consider a non-SACK TCP sender that uses an SMSS of 1460 bytes
and transmits three segments, each with 400 bytes of payload.
This is a case where Early Retransmit could aid loss recovery if
one segment is lost. However, in this case, ER_thresh will
become zero, per Equation (1), because the number of outstanding
bytes is a poor estimate of the number of outstanding segments.
A similar problem occurs for senders that employ SACK, as the
expression "ownd - SMSS" will become negative.

Next, consider a non-SACK TCP sender that uses an SMSS of
1460 bytes and transmits 10 segments, each with 400 bytes of
payload. In this case, ER_thresh will be 2 per Equation (1).
Thus, even though there are enough segments outstanding to
trigger fast retransmit with the standard duplicate ACK
threshold, Early Retransmit will be triggered. This could cause
or exacerbate performance problems caused by segment reordering
in the network.

3.2. Segment-Based Early Retransmit

A TCP or SCTP sender MAY use segment-based Early Retransmit.

Upon the arrival of an ACK, a sender employing segment-based Early
Retransmit MUST use the following two conditions to determine when an
Early Retransmit is sent:

(3.a) The number of outstanding segments (oseg) -- segments sent but
not yet acknowledged -- is less than four.

(3.b) There is either no unsent data ready for transmission at the

sender, or the advertised receive window does not permit new
segments to be transmitted.

When the above two conditions hold and a TCP connection does not
support SACK, the duplicate ACK threshold used to trigger a
retransmission MUST be reduced to:

ER_thresh = oseg - 1 (2)

duplicate ACKs, where oseg represents the number of outstanding
segments. (We discuss tracking the number of outstanding segments
below.) We call this reduced ACK threshold enabling "Early
Retransmission".

When conditions (3.a) and (3.b) hold and a TCP connection does
support SACK or SCTP is in use, Early Retransmit MUST be used only
when "oseg - 1" segments have been SACKed. A segment is considered
to be SACKed when all of its data bytes (TCP) or data chunks (SCTP)
have been indicated as arrived by the receiver.

If either (or both) condition (3.a) and/or (3.b) does not hold, the
transport MUST NOT use Early Retransmit, but rather prefer the
standard mechanisms, including fast retransmit and limited transmit.

This version of Early Retransmit solves the precision issues
discussed in the previous section. As noted previously, the cost is
that the implementation will have to track segment boundaries to form
an understanding as to how many actual segments have been
transmitted, but not acknowledged. This can be done by the sender
tracking the boundaries of the three segments on the right side of
the current window (which involves tracking four sequence numbers in
TCP). This could be done by keeping a circular list of the segment
boundaries, for instance. Cumulative ACKs that do not fall within
this region indicate that at least four segments are outstanding, and
therefore Early Retransmit MUST NOT be used. When the outstanding
window becomes small enough that Early Retransmit can be invoked, a
full understanding of the number of outstanding segments will be
available from the four sequence numbers retained. (Note: the
implicit sequence number consumed by the TCP FIN bit can also be
included in the tracking of segment boundaries.)

4. Discussion

In this section, we discuss a number of issues surrounding the Early
Retransmit algorithm.

4.1. SACK vs. Non-SACK

The SACK variant of the Early Retransmit algorithm is preferred to
the non-SACK variant in TCP due to its robustness in the face of ACK
loss (since SACKs are sent redundantly), and due to interactions with
the delayed ACK timer (SCTP does not have a non-SACK mode and
therefore naturally supports SACK-based Early Retransmit). Consider
a flight of three segments, S1...S3, with S2 being dropped by the
network. When S1 arrives, it is in order, and so the receiver may or
may not delay the ACK, leading to two scenarios:

(A) The ACK for S1 is delayed: In this case, the arrival of S3 will

trigger an ACK to be transmitted, covering S1 (which was
previously unacknowledged). In this case, Early Retransmit
without SACK will not prevent an RTO because no duplicate ACKs
will arrive. However, with SACK, the ACK for S1 will also
include SACK information indicating that S3 has arrived at the
receiver. The sender can then invoke Early Retransmit on this
ACK because only one segment remains outstanding.

(B) The ACK for S1 is not delayed: In this case, the arrival of S1

triggers an ACK of previously unacknowledged data. The arrival
of S3 triggers a duplicate ACK (because it is out of order).
Both ACKs will cover the same segment (S1). Therefore,
regardless of whether SACK is used, Early Retransmit can be
performed by the sender (assuming no ACK loss).

4.2. Segment Reordering

Early Retransmit is less robust in the face of reordered segments
than when using the standard fast retransmit threshold. Research
shows that a general reduction in the number of duplicate ACKs
required to trigger fast retransmit to two (rather than three) leads
to a reduction in the ratio of good to bad retransmits by a factor of
three [Pax97]. However, this analysis did not include the additional
conditioning on the event that the ownd was smaller than four
segments and that no new data was available for transmission.

A number of studies have shown that network reordering is not a rare
event across some network paths. Various measurement studies have
shown that reordering along most paths is negligible, but along
certain paths can be quite prevalent [Pax97, BPS99, BS02, Pir05].
Evaluating Early Retransmit in the face of real segment reordering is
part of the experiment we hope to instigate with this document.

4.3. Worst Case

Next, we note two "worst case" scenarios for Early Retransmit:

(1) Persistent reordering of segments coupled with an application

that does not constantly send data can result in large numbers of
needless retransmissions when using Early Retransmit. For
instance, consider an application that sends data two segments at
a time, followed by an idle period when no data is queued for
delivery. If the network consistently reorders the two segments,
the sender will needlessly retransmit one out of every two unique
segments transmitted when using the above algorithm (meaning that
one-third of all segments sent are needless retransmissions).
However, this would only be a problem for long-lived connections
from applications that transmit in spurts.

(2) Similar to the above, consider the case of that consist of two

segment each and always experience reordering. Just as in (1)
above, one out of every two unique data segments will be
retransmitted needlessly; therefore, one-third of the traffic
will be spurious.

Currently, this document offers no suggestion on how to mitigate the
above problems. However, the worst cases are likely pathological.
Part of the experiments that this document hopes to trigger would
involve better understanding of whether such theoretical worst-case
scenarios are prevalent in the network, and in general, to explore
the trade-off between spurious fast retransmits and the delay imposed
by the RTO. Appendix A does offer a survey of possible mitigations
that call for curtailing the use of Early Retransmit when it is
making poor retransmission decisions.

5. Related Work

There are a number of similar proposals in the literature that
attempt to mitigate the same problem that Early Retransmit addresses.

Deployment of Explicit Congestion Notification (ECN) [Flo94, RFC3168]
may benefit connections with small congestion window sizes [RFC2884].
ECN provides a method for indicating congestion to the end-host
without dropping segments. While some segment drops may still occur,

ECN may allow a transport to perform better with small congestion
window sizes because the sender will be required to detect less
segment loss [RFC2884].

[Bal98] outlines another solution to the problem of having no new
segments to transmit into the network when the first two duplicate
ACKs arrive. In response to these duplicate ACKs, a TCP sender
transmits zero-byte segments to induce additional duplicate ACKs.
This method preserves the robustness of the standard fast retransmit
algorithm at the cost of injecting segments into the network that do
not deliver any data, and therefore are potentially wasting network
resources (at a time when there is a reasonable chance that the
resources are scarce).

[RFC4653] also defines an orthogonal method for altering the
duplicate ACK threshold. The mechanisms proposed in this document
decrease the duplicate ACK threshold when a small amount of data is
outstanding. Meanwhile, the mechanisms in [RFC4653] increase the
duplicate ACK threshold (over the standard of 3) when the congestion
window is large in an effort to increase robustness to segment
reordering.

6. Security Considerations

The security considerations found in [RFC5681] apply to this
document. No additional security problems have been identified with
Early Retransmit at this time.

7. Acknowledgments

We thank Sally Floyd for her feedback in discussions about Early
Retransmit. The notion of Early Retransmit was originally sketched
in an Internet-Draft co-authored by Sally Floyd and Hari
Balakrishnan. Armando Caro, Joe Touch, Alexander Zimmermann, and
many members of the TSVWG and TCPM working groups provided good
discussions that helped shape this document. Our thanks to all!

Appendix A. Research Issues in Adjusting the Duplicate ACK Threshold

Decreasing the number of duplicate ACKs required to trigger fast
retransmit, as suggested in Section 3, has the drawback of making
fast retransmit less robust in the face of minor network reordering.
Two egregious examples of problems caused by reordering are given in
Section 4. This appendix outlines several schemes that have been
suggested to mitigate the problems caused by Early Retransmit in the
face of segment reordering. These methods need further research
before they are suggested for general use (and current consensus is
that the cases that make Early Retransmit unnecessarily retransmit a
large amount of data are pathological, and therefore, these
mitigations are not generally required).

MITIGATION A.1: Allow a connection to use Early Retransmit as long as

the algorithm is not injecting "too much" spurious data into the
network. For instance, using the information provided by TCP's
D-SACK option [RFC2883] or SCTP's Duplicate Transmission Sequence
Number (Duplicate-TSN) notification, a sender can determine when
segments sent via Early Retransmit are needless. Likewise, using
Eifel [RFC3522], the sender can detect spurious Early Retransmits.
Once spurious Early Retransmits are detected, the sender can
either eliminate the use of Early Retransmit, or limit the use of
the algorithm to ensure that an acceptably small fraction of the
connection's transmissions are not spurious. For example, a
connection could stop using Early Retransmit after the first
spurious retransmit is detected.

MITIGATION A.2: If a sender cannot reliably determine whether an

Early-Retransmitted segment is spurious or not, the sender could
simply limit Early Retransmits, either to some fixed number per
connection (e.g., Early Retransmit is allowed only once per
connection), or to some small percentage of the total traffic
being transmitted.

MITIGATION A.3: Allow a connection to trigger Early Retransmit using

the criteria given in Section 3, in addition to a "small" timeout
[Pax97]. For instance, a sender may have to wait for two
duplicate ACKs and then T msec before Early Retransmit is invoked.
The added time gives reordered acknowledgments time to arrive at
the sender and avoid a needless retransmit. Designing a method
for choosing an appropriate timeout is part of the research that
would need to be involved in this scheme.